In general, the rotor of a centrifuge rotating at very high speed is rotationally driven by a flexible shaft surrounded by a sheath. The flexible shaft and the sheath are rotationally mounted in a ball bearing race and joined to a driveshaft, most often the shaft of an electric drive motor, by means of an elastic coupling. As is known, the flexible shaft has the role of limiting the transmission, to the framework of the centrifuge, of the vibrations generated by the rotor and due, for example, to the manufacturing tolerances or to a poor distribution of the load.
It is also known that when the flexible shaft equipped with the rotor is rotating it tends to rotate about its axis of inertia. This axis passes through the center of gravity of the shaft/rotor assembly. More precisely, after a first phase of flexing of the shaft generated by the out-of-balance force of the assembly, the gyroscopic effect generates a righting torque and, as a final result, the axis of rotation passes through the center of gravity of the assembly. This first phase, which is stabilized, is obtained at a speed of rotation of the order of 400 to 1,000 rpm, which, of course, is a function of the shaft, of the rotor, and of the out-of-balance force. This final axis of rotation is radially offset parallel to the geometric assembly axis of the assembly.
It has already been proposed to limit, on the one hand, and on the other hand to damp, this radial offset. This is the case, for example, in the assembly described in U.S. Pat. No. 4,568,324. In this patent, an elastic seal is located between the flexible shaft and the sheath secured to the shaft in order to damp the radial offset. Moreover, the radial offset is limited by a fixed sleeve secured to the framework surrounding the flexible shaft and carrying a ball bearing at an axial distance from the elastic damping seal. The radial offset is limited by the radial clearance existing between the flexible shaft and the internal raceway of the rolling-contact bearing. This assembly exhibits the following drawbacks. On the one hand, when this radial clearance is absorbed, the shaft abuts against the internal raceway of the rolling-contact bearing when the latter is at rest. This gives rise to a rapid deterioration of the shaft. On the other hand, due to the relative arrangement of the rotor, the rolling-contact bearing and the elastic seal, the latter is distant from the rotor, which is the main source of out-of-balance force and vibrations, and damping is not conveniently achieved.
The object of the present invention is to overcome the above described drawbacks.